1. Field of the Invention
The present invention relates to a semiconductor device having a capacitor structure formed by sandwiching a capacitor film composed of a dielectric material between a lower electrode and an upper electrode, and is particularly preferable for application to a ferroelectric capacitor structure in which a capacitor film is composed of a ferroelectric material.
2. Description of the Related Art
In recent years, development of a ferroelectric memory (FeRAM: Ferro-electric Random Access Memory) that holds information in a ferroelectric capacitor structure by using polarization inversion of ferroelectrics has been pursued. A ferroelectric memory especially attracts attention because it is a nonvolatile memory which does not lose information held therein even when the power supply is cut off, and realization of high integration density, high-speed drive, high durability and low power consumption can be expected from it.
A ferroelectric capacitor structure is easily deteriorated in its characteristics by an external hydrogen gas and water. It is also easily deteriorated in its characteristics by etching processing or the like. Therefore, restoration of capacitor characteristics by high temperature annealing is essential. Thus, as the materials of the upper electrodes and the lower electrodes constructing the ferroelectric capacitor structures, noble metals and noble metal oxides capable of resisting high temperature annealing are frequently used as disclosed in, for example, Japanese Patent Application Laid-open No. 2003-174095. As the material of the upper electrodes, an iridium oxide which is a conductive oxide is promising for suppression of the characteristics degradation.
In a ferroelectric capacitor structures, the following two functions are required of the upper electrode in addition to the original role as the capacitor electrode.
(1) The function of forming a favorable interface between a capacitor film and the upper electrode, and obtaining excellent ferroelectric characteristics.
(2) The function of preventing deterioration of the ferroelectric characteristics of the ferroelectric capacitor structure as the manufacturing process advances.
As for the function of (1), an iridium oxide (first iridium oxide film) with high oxidation degree which is more crystallized is formed on the capacitor film. By the first iridium oxide film, excellent ferroelectric characteristics can be obtained.
As for the function of (2), the first iridium oxide film is not preferable as an upper layer in the respect that it is inferior in morphology though it has the above described function, and therefore, an iridium oxide (second iridium oxide film) which is lower in oxidation degree as compared with the first iridium oxide and is not completely crystallized is formed in the upper layer of the first oxide iridium film. The second oxide iridium film is formed into a uniform film excellent in morphology, and therefore, it can suppress degradation of ferroelectric characteristics.
Accordingly, as the upper electrode of the ferroelectric capacitor structure, it is desirable to form it into a construction including a stacked structure of the first iridium oxide film and the second iridium oxide film.
However, when the second iridium film of the upper layer is formed of an iridium oxide in the amorphous state, the iridium oxide is being reduced as the manufacturing process advances though it is formed into a uniform film excellent in morphology at the beginning of film formation. Thereby, a number of voids occur inside thereof, and it becomes an iridium oxide with sparse density, in a so-called porous state. As a result, hydrogen diffusion is promoted through the voids in the second iridium oxide film, and further reduction of the iridium oxide and degradation of the ferroelectric characteristics are promoted.